The present invention relates to friction stir welding and, more particularly, relates to backing up a weld joint during friction stir welding.
Friction stir welding is a relatively new process using a rotating tool to join in a solid state two workpieces or to repair cracks in a single workpiece. At present, the process is applied almost exclusively in straight-line welds. For example, such a process is described in U.S. Pat. No. 5,460,317 to Thomas et al., the contents of which are incorporated herein by reference. During friction stir welding a rotating tool is plunged into a workpiece or between two workpieces by a friction stir welding machine to produce the required resistance force to generate sufficient frictional heating to form a region of plasticized material. Upon solidification of the plasticized material, the workpieces are joined along the weld joint. The magnitude of force exerted by the friction stir welding tool on a workpiece is applied over a small, but well defined area of the workpiece, and must be maintained above a prescribed minimum in order to generate the required frictional heating.
To prevent deformation of a workpiece by the force exerted by the friction stir welding tool and maintain dimensional tolerances, the workpiece must have support behind the weld joint over an area at least as large as the imprint of the welding tool. Additionally, because the frictional heat generated by the welding tool plasticizes the material within the weld joint, the plasticized material must be constrained to prevent the material from flowing out of the weld joint and to provide a weld joint having the desired surface finish. When friction stir welding relatively flat workpieces, the weld joint can be supported by a continuous planar surface, such as a steel plate, positioned underneath the workpieces to be joined.
When friction stir welding workpieces having curvilinear geometries, providing adequate support to the weld joint becomes problematic because the curvilinear geometry makes it more difficult to provide a continuous support surface. Such welds are often necessary when fabricating military and commercial aircraft and rocket fuel tanks. In certain instances, a built-up structure can be secured to the interior surfaces of the curvilinear workpieces prior to friction stir welding. However, weight restrictions and/or design parameters often require a finished assembly having a smooth interior surface. As such, the built-up structure must be removed, for example, by machining, which is time consuming and labor intensive and increases the manufacturing cost of the finished assembly.
In seeking better methods for welding curvilinear geometries, a solid backing wheel has been proposed. For example, referring to FIGS. 1A and 1B, there is illustrated one design of a welding apparatus 11 having a solid wheel 13 for backing a friction stir weld joint between two curvilinear structural members 15, as is known in the art. In order to produce a curvilinear weld joint between a pair of curvilinear structural members 15, the friction stir weld tool 17 is positioned in the joint opposite the solid wheel 13. Assuming ideal structural members 15 without deflection or deformation, the joint between the structural members will meet the solid wheel 13 at a tangent point 19, leaving slender gaps 21 on either side of the wheel directly under the shoulder 17a of the friction stir welding tool 17. As illustrated in FIG. 1B, the friction stir welding tool 17 is typically positioned at an angle a with the plane extending from the center of the solid wheel 13 through the tangent point 19. During friction stir welding, the welding force crushes the gaps between the structural members 15 and the solid wheel 13. As the joint cools from welding temperatures, the joint can retain an instantaneous and permanent set in the shape the structural members 15 were welded, i.e., the curvature of the solid wheel 13, which can result in a distorted joint that adversely affects the tolerances of the resulting structural assembly.
In addition, referring to FIG. 1B, the curvature of the solid wheel 13 typically results in some amount of material 23 being extruded through the joint away from the shoulder 17a of the friction stir welding tool 17. As the material 23 is extruded, the shoulder 17a of the friction stir welding tool 17 forms a trough 25 on the side of the joint facing the shoulder, which can weaken the joint. When the centerline of the friction stir welding tool 17 is in line with the center of the solid wheel 13, the tool is aligned at the top dead center (xe2x80x9cTDCxe2x80x9d) of the wheel. Referring to FIGS. 1C and 1D, if the tool 17 is ahead or behind of TDC of the wheel, the tool receives less support on the slope of the wheel. Both cases cause extrusion of material 23 away from the shoulder 17a of the tool 17. As illustrated in FIG. 1C, in the worst case the tool is ahead of TDC and the extrusion 23 builds a xe2x80x9crampxe2x80x9d that wedges under the tangent point of the wheel 13 and lifts the structural members 15 away from the wheel. If the tool 17 is ahead of TDC, the shoulder 17 will also dig a relatively deep trough 25 in the surface of the structural members 15 facing the shoulder. Where the tool is behind TDC, the shoulder 17a extrudes metal 23 away from the shoulder, but the extent of distortion of the structural members 15 is typically not as severe. Reducing the diameter of the solid wheel 13 typically increases the distortion of the structural members 15, as well as the extrusion 23 of material from the backside of the joint, since the slope of the wheel becomes steeper. Increasing the diameter of the solid wheel 13 results in heavier, more complex tooling and can, in certain instances, result in difficulty extracting the wheel from the resulting structural assembly.
Referring to FIGS. 1E and 1F, where there is illustrated a comparison of a friction stir weld joint 27a formed by welding flat plates 15a with a flat backing and a weld joint 27b formed by welding curvilinear plates 15b using a solid backing wheel. As illustrated in FIG. 1E, the backside of the weld joint 27a of the flat plates 15a is flush with the original profile of the structural members 15. In contrast, as illustrated in FIG. 1F, the weld joint formed between the curvilinear members 15 exhibits extrusion 23 on the backside of the joint away from the shoulder and a severe depression or trough 25 on the side of the joint facing the shoulder.
Thus, there is a need for an improved backing device for weld joints formed by friction stir welding workpieces having curvilinear geometries. The backing device should be capable of effectively supporting a weld joint and constraining the plasticized material within the weld joint during friction stir welding. Additionally, the backing device should be easily adaptable to varying workpiece geometries.
The present invention provides an apparatus and an associated method for backing up weld joints formed by friction stir welding workpieces having curvilinear geometries. The backing device includes a backing ring defining a contact zone. Advantageously, the backing ring has sufficient flexibility such that at least a portion of the contact zone defined by the backing ring engages the weld joint opposite the friction stir welding tool to thereby support the weld joint and to constrain the plasticized material within the weld joint.
The backing device also includes a restraining member in operable communication with the backing ring to urge the central axis of the backing ring and, thus, the contact zone of the backing ring, toward the weld joint. According to one embodiment, the restraining member is in operable communication with the backing ring through a spring member and a plurality of bogies. The plurality of bogies rotatively engage at least a portion of the backing ring and urge the contact zone of the backing ring toward the weld joint. The spring member at least partially receives the plurality of bogies and is in operable communication with the retaining member. The spring member urges the bogies toward the backing ring.
According to one embodiment, the backing ring defines an opening therethrough. Advantageously, the backing device can include at least one jet for injecting coolant fluid into the opening in the backing ring to convectively transfer heat from the weld joint. The backing device can also include a retaining roller extending at least partially through the opening defined in the backing ring opposite the bogies. The retaining roller rotatively engages at least a portion of the backing ring such that the backing ring is slidably retained between the bogies and the retaining roller. The retaining roller may comprise one or more bogies.
The backing device includes a sensor, such as a strain-gage load cell, a piezoelectric load cell, a dynamometer, a pneumatic load cell, or a hydraulic load cell, for measuring the magnitude of force exerted by the contact zone of the backing ring upon the weld joint. According to one embodiment, the apparatus also includes a computing means in electrical communication with the sensor.
Advantageously, the backing device also includes means, responsive to the sensor and in operable communication with the restraining member, for moving the restraining member relative to the weld joint in order to modify the force exerted by the contact zone of the backing ring upon the weld joint. According to one embodiment, the moving means responsive to the sensor includes a drive assembly and a motor in operable communication with the drive assembly. The drive assembly can include a belt drive or a gear drive. According to another embodiment, the moving means comprises an actuator assembly, such as a pneumatic actuator arm or a hydraulic actuator arm. In still another embodiment, the moving means includes a power screw.
The backing device can also include means for rotating the backing ring to thereby maintain the contact zone of the backing ring opposite the friction stir welding tool during formation of the weld joint. According to one embodiment, the rotating means includes a drive assembly and a motor in operable communication with the drive assembly. As before, the drive assembly can include a belt drive or a gear drive.
The present invention also provides an apparatus for friction stir welding weld joints in workpieces having curvilinear geometries. According to one embodiment, the friction stir welding device includes a milling machine having a spindle in rotatable communication with a friction stir welding tool, which frictionally engages a workpiece to thereby form a weld joint. The friction stir welding device also includes a backing ring having a central axis and defining a contact zone. Advantageously, at least a portion of the contact zone defined by the backing ring engages the weld joint opposite the friction stir welding tool to thereby provide support behind the weld joint and to constrain the plasticized material within the weld joint.
The friction stir welding device includes at least one sensor for measuring the magnitude of force exerted upon the workpiece. The friction stir welding device can include a computing means in electrical communication with the at least one sensor, such as a strain-gage load cell, a piezoelectric load cell, a dynamometer, a pneumatic load cell, or a hydraulic load cell. According to one embodiment, the at least one sensor includes a first sensor for measuring the magnitude of force exerted by the contact zone of the backing ring upon the weld joint and a second sensor for measuring the magnitude of force exerted by the friction stir welding tool upon the workpiece.
Advantageously, the friction stir welding device also includes means, responsive to the at least one sensor and in operable communication with the backing ring, for moving the central axis of the backing ring relative to the weld joint in order to modify the force exerted by the contact zone of the backing ring upon the weld joint. According to one embodiment, the moving means responsive to the at least one sensor includes a drive assembly and a motor in operable communication with the drive assembly. The drive assembly can include a belt drive or a gear drive. According to another embodiment, the moving means comprises an actuator assembly, such as a pneumatic actuator arm or a hydraulic actuator arm. In still another embodiment, the moving means includes a power screw.
The friction stir welding device can also include means for rotating the backing ring to thereby maintain the contact zone of the backing ring opposite the friction stir welding tool during formation of the weld joint. According to one embodiment, the rotating means includes a drive assembly, such as a belt drive or a gear drive, and a motor in operable communication with the drive assembly.
The present invention also provides a method of friction stir welding a workpiece, comprising the steps of mounting a friction stir welding tool to a rotatable spindle such that the friction stir welding tool rotates with the spindle. The friction stir welding probe is then positioned adjacent to a first side of a workpiece. A backing ring defining a central axis is positioned adjacent to a second side of the workpiece opposite the friction stir welding tool. The friction stir welding tool is then inserted into the first side of the workpiece to form a weld joint. Concurrently with the inserting step, the central axis of the backing ring is moved toward the weld joint such that the backing ring defines a contact zone at least partially engaging the weld joint opposite the friction stir welding tool. The magnitude of the force exerted upon the workpiece is measured. According to one embodiment, the measuring step includes measuring the force exerted by the friction stir welding tool upon the workpiece and measuring the force exerted by the contact zone of the backing ring upon the weld joint. The distance between the central axis of the backing ring and the weld joint is then adjusted to modify the force exerted by the contact zone of the backing ring on the weld joint. The friction stir welding tool is moved through the workpiece. Concurrently with the moving step, the backing ring is rotated along the second side of the workpiece such that the contact zone of the backing ring remains opposite the friction stir welding tool. According to one embodiment, coolant fluid is injected into an opening defined by the backing ring to convectively transfer heat from the weld joint. In another embodiment, at least one bogie is moved relative to the backing ring to thereby modify the force exerted by the contact zone of the backing ring upon the weld joint.
Accordingly, there has been provided an apparatus and associated method allowing for the formation of uniform weld joints in workpieces having curvilinear geometries. In particular, the backing device is capable of effectively supporting a weld joint to thereby maintain the manufacturing tolerances of the workpiece, as well as to constrain the plasticized material within the weld joint. Additionally, the backing ring of the backing device is sufficiently flexible so that it can easily adapt to varying workpiece geometries.